Integrated circuit chips are manufactured in a process whereby hundreds of such identical integrated circuits are provided on a single silicon wafer. The integrated circuits are manufactured by a series of "etches", whereby each integrated circuit is covered with a photo-sensitive etch-resistant material (resist), and then light is projected onto each integrated circuit through a photographic stencil, or mask. The exposed resist is then removed chemically (etched) in a chemical solution, leaving a pattern which duplicates that of the mask. Periodically, dopants to control conductivity and sputtered metal to provide interconnection between various layers, are placed onto the integrated circuit.
After each masking step, regardless of its function, excess or undesired material must be etched from the integrated circuit. Because the etching process is a removal of discrete compounds from the exposed surface of the integrated circuit, the etching material accumulates these particulates removed from the surface. Because of the extremely small circuits involved and the small differences in electrical conductivity of the various portions of an integrated circuit, near-absolute purity in the manufacture of such products is required. Any particulate contamination can have extremely adverse affects on the reliability of an integrated circuit. Therefore, extreme measures are undertaken to prevent particulate contamination during the manufacturing process.
Depending upon the particular requirements, an integrated circuit contained on a silicon wafer may undergo as many as twenty or more individual etching steps. Each etch will occur in a different etching solution, to be followed by a rinse in a deionized water bath. The rinse and deionized water baths are arranged in pairs, with, typically, three pairs together in a single hood. Many such hoods are arranged beside one another in a fabrication area. As noted above, the etching step removes material from the integrated circuit, and over time will accumulate in the etch solution and deionize water rinse bath. Both the etch and rinse baths are filtered to remove as much as possible of this particulate contamination. Periodic testing of each of these solutions is required so as to determine if an unacceptable level of contamination exists in either the etch solution or the rinse solution, so that contaminants are not added to the integrated circuit by either of the solutions.
Heretofore, measurement of contamination in the fabrication process was performed relatively infrequently because of the physical limitations of the sampling apparatus available. Typically, contamination of the acid or deionized water was measured when delivered in bulk, and was not subsequently measured in situ during the fabrication process. The obvious problem resulting from this procedure was that if a contamination problem existed, it went undetected for a substantial period of time, thereby resulting in the loss of product produced during such period of excess contamination. An alternative to no testing for contamination whatsoever was a mobile testing unit which could be moved throughout a fabrication area, with samples being manually withdrawn from each acid or deionized water tank for analysis. In addition to the disruption resulting from this process (the station being sampled had to be shut down in order to manually sample each of the baths), such procedure was extremely time consuming and in a large fabrication area each bath may only have been sampled once every few days or weeks. Obviously, excess contamination could therefore ruin a number of days production before discovery.